I probably have a couple thousand tied for first place and ten thousand tied for second place, but the Algerian 1949 50 F always comes to mind when thinking about it. It is one of the earliest moderns and is quite scarce in unc. It's a bridge between the classics and moderns. It has a classic design and was made to the old standards of quality, but it was issued, circulated, and destroyed like a modern.
Great design indeed. Unfortunately the mintage is only 2000 pieces ... oops, 2007. I do not have one favorite coin either, but many. Like this one from Poland: Christian